This was 15 whole years ago, and maybe times have changed and prices and inflation may affect things now, but for 3 of us we rented a car in Lisbon (think this was Hertz, but might have been another rental company -- we looked at several options before making our decision), and turned it in in Spain something like a week and a half earlier. We paid something close to $600 extra for the open-jaw convenience, not having to return the car within Portugal, find our way somehow across the Spanish border (with luggage) to a place they could rent us anther car, then eventually turn in a second, Spanish rental car in Spain.
So it wasn't fee free, but at the time it was worth not having to back-track through Portugal before heading to Spain, and although it strangely took a really long time (over an hour at the counter) to pick up our pre-reserved-and-paid-for car in Lisbon, the rest of the process went smoothly. Having our own wheels let us set our own itinerary and schedule, including seeing Sintra. Obidos, and Evora in Portugal, then on to Arcos de la Frontera and other places in Spain. This was back in 2002, and your situation might be different now, but for better or worse, it has been done.